Peptide-based R-electron scaffolds offer a unique ability to encourage exciton coupling among i-conjugated subunits in aqueous environments (Ashkenasy et al., 2006; Krieg et al., 2009; Chen et al., 2010; Shao et al., 2010; Sun et al., 2011; Tian et al., 2012; Kas et al., 2008; Kumar et al., 2011; Diegelmann et al., 2008; Schillinger et al., 2009; Stone et al., 2009; Shaytan et al., 2011; Wall et al., 2011; Mba et al., 2011; Vadehra et al., 2010). These scaffolds promote delocalized electronic states among the component conjugated oligomers and, due to their peptidic nature, offer an enticing segue into biological investigations. Current state of the art consists of molecular structures, such as synthetic polypeptides or genetically modified α-helical proteins, which position π-conjugated oligomers with defined spatial orientations, thereby leading to collective electronic delocalization among otherwise isolated electronic units (Kas et al., 2008; Kumar et al., 2011). Likewise, π-electron peptidic materials can be directed to aggregate into nanostructured materials with tube or tape-like morphologies (Diegelmann et al., 2008; Schillinger et al., 2009; Stone et al., 2009; Shaytan et al., 2011; Wall et al., 2011; Mba et al., 2011; Vadehra et al., 2010). Many of the common synthetic approaches to install the requisite π-electron units involve solution-phase reactions between the π-conjugated segments and the peptide fragments, using reactive functional groups, such as amines or carboxylic acids (for amide bond formation) or alkynes (for Huisgen-type cycloadditions). These examples, however, require the up-front chemical synthesis of the conjugated oligomer of interest appended with the necessary reactive groups to allow for ligation onto or within the peptide framework, thus posing challenges for mutual peptide/chromophore solubility and for final construct purification.
An alternative synthetic strategy was recently reported that keeps the peptides bound to solid supports during the installation of the π-electron segments via site-site double amidation between immobilized peptides and π-conjugated diacids (Vadehra et al., 2010). This approach still requires the synthesis of the diacid components, and the solubilities of the critical π-electron segments become problematic as the conjugated oligomer is made longer.